There is a real need for efficient photocathodes which operate in the low frequency end of the visible spectrum and in the near infrared. Image intensifiers using such photocathodes permit viewing under moonlight and starlight conditions with a clarity that approaches broad daylight. A particularly useful cathode for this purpose is a monocrystalline p-type gallium arsenide or gallium phosphide epitaxial layer grown on a monocrystalline substrate of the same type material. In an effort to extend the spectral response to cover both invisible and visible regions, mixtures of compounds have been employed. A particularly promising combination is an In.sub.x Ga.sub.1-x P substrate with a semitransparent In.sub.y Ga.sub.1-y As photocathode. The substrate is grown on GaP, usually by the vapor hydride method of deposition. To avoid lattice mismatch between the substrate and the GaP host crystai the number of indium atoms in the depositing atmosphere is increased from zero at the beginning of the deposition of the substrate to the level required to deposit x atoms in the stochiometric formula above, where x varies directly and linearly with the thickness of the substrate from zero ot its final value.
While this satisfies the physical requirements for a stable structure and provides the desired dynamic characteristics of broad spectral response, the electron image contains an undesireable cross-hatch pattern which reduces the resolution of image intensifiers in which the cathode is employed. There is also a percentage of growth protuberances, called hillocks, which reduce the resolution in random areas of the image.